Add a reusable chain_id query-param parser to the common library

[CryptoFewka]

Context

Raised in review of optimum-measurements #66 by @abergasov (comment): a chain_id query-param parser written for the measurements HTTP handlers is likely useful beyond that one service, so it should live in optimum-common and be reused.

What measurements has today (to be replaced by the shared helper)

Two small parsers for the chain_id query param, differing only in how they treat 0:

• internal/routes/block_latency.go parseChainIDValue(raw string) (uint64, bool) — empty means "all chains" (0); a present value must be a non-zero uint64; 0 or malformed is rejected.
• internal/routes/operator_mump2p_first_seen.go parseOptionalChainID(raw string) (uint64, error) — empty means "all chains" (0); a literal 0 is allowed; present values parse as uint64.

Proposal

Add a reusable chain-id query-param parser to optimum-common (candidate homes: pkg/net for HTTP query helpers, or pkg/chain). It should cover both semantics above, e.g. a single function with an "allow zero" option, or two clearly-named variants:

• "all-chains-or-specific, zero is the all-chains sentinel and not a valid explicit value" (block_latency semantics), and
• "all-chains-or-specific, zero is an acceptable explicit value" (mump2p semantics).

Then have optimum-measurements adopt it and delete the two local copies (and fold the existing unit tests into the common package).

Acceptance

• Helper(s) + table tests in optimum-common.
• optimum-measurements uses the shared helper in both handlers; local parseChainIDValue / parseOptionalChainID removed.

Written with Claude Code

[linear-code]

OPT-866